In recent years, it has become commonplace for a computer located at a central station to use the telephone network to automatically contact remote computers to either receive data from the remote computers or supply data to the remote computers. For example, the headquarters of a store chain may contact, on a daily or weekly basis, each of the chain's stores to obtaining gross receipts information and information about the numbers of specific items sold during the preceding day or week so that product inventories can be rapidly and quickly replenished. In most instances, such contacts are usually automatically made through modems with dedicated telephone lines when telephone company rates are low, i.e., during the evening hours.
Previously developed systems include a modem located at the central station for coupling the central station's computer to a telephone company (i.e., telco) line. Among other things, the computer sequentially generates the signals necessary to cause the modem to automatically dial the telephone numbers of remote stations. After dialing the telephone number of a remote station, the modem may also transmit a security code or hang up and have the remote station call back on a predetermined telephone number.
If the central station and the remote station are both using dedicated telco lines, the central station's computer is connected to the remote station as soon as the remote station modem responds to the call from the central station by sending an answer tone. While such systems are satisfactory, they have one significant disadvantage. They are expensive because they rely on expensive, dedicated lines, i.e., central office telephone lines that are only used for making telephone calls from the central station to the remote stations. Dedicated lines are located on both ends of the system--between the central station and its central office and between each remote station and its related central office.
In an effort to avoid the need for dedicated lines, attempts have been made to use shared lines and switches at the remote stations connect central station computers to remote stations. In switched systems the central station modem is required to repeat an access code such as *7, *77, etc., for a predetermined period of time fixed by the central station's computer. During the fixed period of time, a telephone connection is made to the remote station and the remote station sends an answer tone to the central station.
The central station must wait until the end of the fixed period of time before beginning data communication because data communication cannot take place until the access code stops. In order to make sure all remote stations are contacted, the fixed period of time must be the greatest period of time required to access any given station. Telephone contact time between a calling station and a called station is random, usually falling within a 3- to 32-second range. The average time to make contact usually falls in the middle of this range, i.e., around 16 seconds. As a result, on an average, 16 seconds of data communication time are lost each time a central station "dials" a remote station using a switching device. While, on an individual basis, 16 seconds is relatively short, this interval rapidly adds up when a central station seeks to contact several hundred or several thousand remote stations in an evening. In such cases, 16 seconds per call in lost data transmission time can cost several hundred dollars per day in unnecessary long-distance telephone charges.
Unfortunately, a single access code transmission on a shared line is unsatisfactory because, in the past, shared line loop activity can cause incorrect modem operation. The present invention is directed to avoiding unnecessary telco charges and improving data communication by providing a polling controller that controls telephone contact between a central station modem and a telco line such that data communication commences immediately after a telephone connection is made between the central station and a remote station.